Indirect heated hot water systems

ABSTRACT

An indirect hot water system is provided with a secondary heat exchanger ( 130 ) that heats potable water upstream of the main heating tank ( 110 ). The secondary heat exchanger utilises refrigerant that has passed through the main heating tank. The system includes clips ( 40, 42, 60, 62 ) that support and separate the coils of the heat exchangers ( 108, 116 ) in the main tank ( 110 ) to increase the efficiency of the heat exchangers and the system overall. A pressure control valve maintains a substantially constant pressure and limits refrigerant flow in the secondary heat exchanger to maintain a high refrigerant temperature when the system is not operating at full capacity.

FIELD OF INVENTION

This invention relates to indirect water heating systems and more particularly to improvements in efficiency, longevity and usability of such systems.

BACKGROUND

Indirect water heating systems do not heat the potable water directly but utilise one or more intermediate fluids to transfer heat from a heat source to the potable water. Typical heat sources include heat pumps and solar panels, although gas burners are also used. Typical intermediate fluids include refrigerant gases, water and glycol.

A typical installation may utilise a heat pump to both cool air and provide hot water.

Normal indirect water heating systems include an open topped tank that contains a body of static fluid, usually water. Located in the tank are two coils, one carrying potable water to be heated and the other carrying a heating fluid heated by the heat pump. The heated fluid transfers some of its heat to the static fluid and in turn the static fluid transfers some of its heat to the potable water.

When providing hot water, the static fluid is usually heated to a temperature in the range of 65° C. to 85° C. This means the heating fluid leaves the tank at about this temperature and so, without additional cooling, the difference in temperature between the inlets and outlets of the heat pump is low. The temperature drop of the heating fluid within the water tank is also relatively low, so the amount of heat transferred from the heating fluid to the static fluid is quite low.

In addition, where the system provides a cooling function as well as a heating function, it is desirable to reduce the temperature of the refrigerant fluid to as low a temperature as practicable before expanding it to cool air or water. A high temperature of the fluid at entry to an expansion valve results in a higher temperature after expansion and correspondingly less cooling effect. This can be overcome by cooling the refrigerant between the water tank and the expansion valve, but this results in heat being discharged to the environment and wasted.

Existing indirect water heating tanks typically utilise plastics pipes for the potable water heat exchanger without anything to hold and separate adjacent windings of the coils. The lack of separation and the use of plastics reduce heat transfer rates. The coils usually have no permanent support and so a loose and unwieldy coil of plastics pipe in the tank complicates any maintenance. Copper pipes have been used but typically with spacers welded to the tube walls. Over time, the thermally generated expansion and contraction of the tubing and spacers causes the wall of the tubing to fail.

Indirect water heating systems heat the potable water as it flows through the heat exchanger. Low rates of heat transfer limit the maximum flow rate of the potable water. Lowering the temperature of the refrigerant more than otherwise using the potable water results in more heat transferred to the potable water and more heat transferred, so raising the overall efficiency of the system; less heat is wasted by being transferred to the ambient environment.

Existing open topped indirect hot water heating tanks have a lid that is sealed against the tank. A gasket is provided to aid in sealing and this is attached to the tank. The current sealing arrangements are not satisfactory as when raising the lid, the gasket frequently adheres to the lid and detaches from the tank. Attachment of the gasket to the tank is not easy.

SUMMARY OF INVENTION

In one broad form the invention provides a heat exchange system that transfers heat between a first fluid and a second fluid, the system including:

-   -   a first heat exchanger including:     -   a first inlet for the first fluid;     -   a second inlet for the second fluid;     -   a first outlet for the first fluid;     -   a second outlet for the second fluid; and     -   a constant pressure valve located downstream of the first heat         exchanger and having a fifth inlet in fluid communication with         the first outlet of the first heat exchanger for maintaining a         substantially constant pressure at the fifth inlet.

The system may have a second heat exchanger including:

-   -   a third inlet for the first fluid;     -   a fourth inlet for the second fluid;     -   a third outlet for the first fluid;     -   a fourth outlet for the second fluid,     -   wherein, the third inlet is located downstream of the first         outlet and the second inlet is located downstream of the fourth         outlet.

The system may be configured so that the third inlet receives first fluid from the first outlet and the second inlet receives second fluid from the fourth outlet.

The constant pressure valve is preferably located downstream of the third outlet.

The first heat exchanger may be an indirect heat exchanger utilising an intermediate fluid to transfer heat between the first and second fluids.

The second heat exchanger may be a reverse flow heat exchanger.

The first heat exchanger may include:

-   -   a tank;     -   a third fluid within the tank;     -   at least one first pipe extending between the first inlet and         first outlet and extending at least partially within the third         fluid, and     -   at least one second pipe extending between the second inlet and         second outlet and extending at least partially within the third         fluid.

In another broad form the invention also provides a heat exchange system that transfers heat between a first fluid and a second fluid, the system including:

-   -   a first heat exchanger including:         -   a tank;         -   a third fluid within the tank;         -   a first inlet for the first fluid;         -   a second inlet for the second fluid;         -   a first outlet for the first fluid;         -   a second outlet for the second fluid;         -   at least one first pipe extending between the first inlet             and first outlet and extending at least partially within the             third fluid, and         -   at least one second pipe extending between the second inlet             and second outlet and extending at least partially within             the third fluid; and     -   a second heat exchanger including:         -   a third inlet for the first fluid;         -   a fourth inlet for the second fluid;         -   a third outlet for the first fluid;         -   a fourth outlet for the second fluid,     -   wherein, the third inlet receives first fluid from the first         outlet and the second inlet receives second fluid from the         fourth outlet.

The system preferably includes a constant pressure valve located downstream of the first heat exchanger and having a fifth inlet in fluid communication with the first outlet of the first heat exchanger for maintaining a substantially constant pressure at the fifth inlet.

More preferably the constant pressure valve is located downstream of the third outlet.

In a further broad form the invention also provides a heat exchange system that transfers heat between a first fluid and a second fluid, the system including:

-   -   a first heat exchanger including:         -   a first inlet for the first fluid;         -   a second inlet for the second fluid;         -   a first outlet for the first fluid;         -   a second outlet for the second fluid; and     -   a second heat exchanger including:         -   a third inlet for the first fluid;         -   a fourth inlet for the second fluid;         -   a third outlet for the first fluid;         -   a fourth outlet for the second fluid,         -   the third inlet receiving first fluid from the first outlet             and the second inlet receiving second fluid from the fourth             outlet, and         -   a constant pressure valve located downstream of the first             heat exchanger and having a fifth inlet in fluid             communication with the first outlet of the first heat             exchanger for maintaining a substantially constant pressure             at the fifth inlet.

The constant pressure valve is preferably located downstream of the third outlet.

The first heat exchanger is preferably an indirect heat exchanger utilising a third fluid to transfer heat from the first fluid to the second fluid.

In preferred embodiments the first and second fluids preferably flow through at least one first and at least one second pipe, respectively, in the first heat exchanger.

Preferably the at least one first pipe is coiled. The at least one second pipe is preferably arranged in an elongate helix.

The system may include at least one spacer clip attached to the coils of the at least one second pipe, the or each said at least one spacer clip attaching to at least one pair of coils to separate the coils from each other.

The at least one second pipe may include at least two concentric elongate coils and the at least one spacer clip includes at least one spacer clip attached to at least one coil of at least two of said at least two concentric coils to separate the coils from each other.

The at least one spacer clip may include a body having at least two recesses adapted to receive a pipe and to grip the pipe.

Preferably each recess has an interior portion and an opening, the opening sized to require deformation of the recess to receive a pipe within the recess. The recess may be generally circular or rectangular. The recess may be rectangular and have a back wall and two, generally opposed side walls extending from the back wall The sidewalls may be substantially parallel or may diverge away from the back wall.

The clips may be comprised of bent wire or rod.

The system may also include at least one hollow reservoir within the first tank, the interior of at least one reservoir being in fluid communication with at least one of the second inlet and the second outlet.

In a yet further broad form the invention provides an indirect hot water heating apparatus including:

-   -   a tank having an opening;     -   a lid that engages the opening of the tank and     -   a gasket sandwiched between the tank and the lid,     -   wherein the opening is defined by at least one wall of the tank,         the wall defining part of an annular groove that extends         outwardly from adjacent the opening, the gasket having a         complementary annular tongue that engages the annular groove.

Preferably the annular groove extends outwardly.

The tank may have an inner wall and an outer wall surrounding the inner wall and wherein the inner wall has a first annular portion extending outwardly from a free end portion and the outer wall has a second annular portion extending inwardly toward the inner wall adjacent the first annular portion, said first and second annular portions defining opposing walls of the groove.

The tank may have inner wall has a first annular portion extending outwardly from a free end portion and a second annular portion extending outwardly adjacent the first annular portion, said first and second annular portions defining opposing walls of the groove.

In a yet another broad form the invention provides a clip for separating windings of a heat exchanger coil, the clip having a series of spaced apart recesses, each adapted to engage part of each winding and to maintain adjacent windings separated.

Preferably at least one of the recesses is generally part circular and preferably at least one of the recesses has an arcuate surface that extends for more than 180 degrees. Most preferably at least one of the recesses has an arcuate surface that extends for more than 300 degrees.

At least one of the recesses may have a generally rectangular shape.

At least one of the recesses may be defined by a rear surface, two spaced apart and opposed side surfaces and an opening, the side surfaces extending from the rear surface to the opening. The side surfaces may be parallel or may diverge from each other away from the rear surface.

The invention will be better understood from the following non-limiting description of preferred embodiments of the invention and the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a typical prior art indirect water-heating tank;

FIG. 2 is a schematic diagram showing an indirect hot water heating system according to a first embodiment of the invention;

FIG. 3 is a side view of a first spacer for use with tubing, such as in an indirect hot water heating tank;

FIG. 4 is a side view of a second spacer for use with tubing, such as in an indirect hot water heating tank;

FIG. 5 is a side view of a third spacer for use with tubing, such as in an indirect hot water heating tank;

FIG. 6 is a side view of a fourth spacer for use with tubing, such as in an indirect hot water heating tank;

FIG. 7 is a plan view from above of an indirect hot water heating tank with its lid removed showing the arrangement of the coils;

FIG. 8 is an axial cross-sectioned view of a part of the tank of FIG. 7.

FIG. 9 is a cross sectional view of an indirect hot water heating tank according to another aspect of the invention;

FIG. 10 is a plan view from above of the tank of FIG. 9.

FIG. 11 is a cross sectional view of the top edge of the tank of FIG. 9 according to a first embodiment.

FIG. 12 is a cross sectional view of the top edge of the tank of FIG. 9 according to a second embodiment.

FIG. 13 is a schematic diagram showing an indirect hot water heating system according to a further embodiment of the invention.

FIG. 14 is a schematic diagram showing an indirect hot water heating system according to a further embodiment of the invention.

FIG. 15 shows a cross section through a tank having an electrically operated water level sensor.

DETAILED DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

Referring to FIG. 1 there is shown a typical indirect hot water heating tank 10. The tank 10 is normally cylindrical, but may the square, rectangular or oval when viewed from above. The tank is not permanently sealed but is provided with a removable lid 14. The lid 14 is attached to the body 12 of the tank 10 with fasteners (not shown), with the joint therebetween sealed with a gasket 16.

Located within the tank 10 are a body of static fluid 18, usually water, coils 20 carrying potable water and a coil 22 carrying a heating fluid. The heating fluid passes through the coil 22, heating the static fluid 18, which in turn heats potable water as it flows through the coil 20.

The coil 20 is typically made of polyethylene or polypropylene plastics materials and, as can be seen, is typically not provided with any support structure. This results in the spirals of the coil resting on each other and so reducing the effective area for heat transfer and movement of water past the coils. The tank 10 may also include an electric heater 19 that is used to boost the heating of the static water.

Referring to FIGS. 3 to 6 there are shown a number of coil spacers that may be utilised to support and separate the coils of a heat exchanger, such as used in an indirect water-heating tank.

The spacers 40 and 42 of FIGS. 3 and 4 respectively are intended for use with either copper or plastics pipe. The spacers comprise a piece of stiff wire or rod bent, preferably formed of stainless steel, into the shapes shown. Each spacer has a series of part circular holding portions 44 separated by a spacing portion 46 between adjacent spacing portions. The holding portions are for receiving the tubing of a heat exchanger coils and extend about an arc for approximately 300 degrees. This leaves a throat or opening 48 that is slightly smaller than the outside dimeter of the pipe 49 used. The length of the spacing portion is such to give a suitable separation 51 between the tube centres. When used with 19.05 mm outside diameter pipe, a spacing 51 between centres of 30 mm may be used. Closer or wider spacings may be used. With 19 mm outside diameter pipe the opening 48 is about 18.5 mm.

The pipe 49 is easily inserted into the holding portions. The wire is chosen of a material and diameter so that it will flex and expand the opening 48 to allow a relatively rigid pipe, such as copper pipe, to be placed within the holding portion. When a relatively flexible pipe, such as a plastics pipe is used, the pipe may flex rather than the spacer or both the spacer and the pipe may flex.

The first spacer 40 is designed to be attached vertically to the coils to space the windings vertically. Typically a circular coiled heat exchanger will have four spacers attached, arranged at 90° to each other around the circumference. More than four vertical spacers may be used, particularly for an oval shaped coil. When a heat exchanger has three or more such spacers attached to the coils, relative movement of the coils to each other is effectively prevented in both the vertical and horizontal directions and a single coil heat exchanged may easily be manipulated. Where the heat exchanger is oval it will be appreciated that extra spacers 40 may be used. It will be noted that the lower end 50 of the spacer 40 includes a horizontal foot 52 that in use rests on the base of the heating tank. The foot 52 extends underneath the tubing and so does not increase the horizontal space require for the coil. The feet of the spacers thus provide some extra stability to the coil.

The second spacer 42 is designed where the exchanger has multiple, concentric windings. In such a situation each coil will have its own set of vertical spacers 40. The concentric coils are then separated from each other by sets of horizontal spacers 42 attached to the uppermost and lowermost portions of the coils. This holds the concentric coils away from each other in a relatively rigid manner.

FIGS. 5 and 6 show a second set of spacers, 60 and 62, respectively. The spacer 60 is a vertical spacer and functions in a similar manner to the spacer 40 of FIG. 3. Spacer 62 is a horizontal spacer and functions similarly to the spacer 42 of FIG. 4.

The spacer 60 is formed with a generally rectangular holding portion 64 separated by spacing portions 66 between adjacent holding portions. Each holding portion includes two generally opposed sections 68 and a generally transverse section 70 therebetween. The two opposed sections 68 extend at substantially 900 to the transverse section 70 and are substantially parallel to each other. The length of the transverse section 70 is such that the separation 72 of the two opposed sections is less than the diameter of the tubing being used. If copper tubing is being used, the spacing is only slightly less than the outside diameter of the tubing. To mount the spacer on the coil one merely pushes the tubing into the opening 74, causing each of the two horizontal sections to flex and/or bend at the junction with the vertical portion. The separation 72 is chosen to allow easy insertion of the tubing whilst providing sufficient grip due to compressive forces on the tube to hold it in the spacer.

When used with tubing of plastics materials, the opening 74 may need to be appreciably less than the outside diameter of the tube to provide sufficient grip, due to the greater flexibility of the tubing.

The spacer 62 shown in FIG. 6 differs slightly from the spacer 60 of FIG. 5. The spacer includes holding portions 75, each of which includes opposed side arms 76 between the back section 78 and the spacing portions 80. The two side arms 76 are angled toward each other so the opening 82 is smaller than the back portion 78. This aids in retaining a tube in the holding portion 75, as removal of the tube from the opening requires the two arms 76 to be bent away from each other to increase the size of the opening 82. This configuration is relatively easier to manufacture than the arrangement of FIGS. 3 and 4 whilst providing a similar functionality.

It will be appreciated that the arrangement of the holding portions of the spacers 60 and 62 may be applied to the other device.

It will be appreciated that the number of holding portions on each spacer will be chosen to suit the heat exchanger and are not limited to the numbers shown in the drawings. It will be appreciated that the spacers may be made of materials other than metal and that a wire configuration is not necessary. For example, the spacers may be made of a relatively stiff sheet of plastics material with the holding recesses formed in an edge of the sheet.

Referring to FIG. 7 there is shown a plan view of an oval shaped open topped hot water tank 80. The tank has five heat exchanger coils 82 arranged concentrically about the outside portion within the tank. The five coils may be for a single heat exchanger, such as the potable water heat exchanger or the refrigerant heat exchanger or may be for both heat exchangers. The coils are secured together by horizontal clips 84 and vertical clips 86. For clarity only one set of vertical clips 86 is shown. There are a series of clips 86 located around the sets of coils. The horizontal clips 84 may be of the types of FIGS. 3 or 5 and the vertical clips 86 may be of the type of FIGS. 4 or 6.

FIG. 7 also shows a two potable water storage tanks 88 located within the tank 80. The tanks are cylindrical and extend the height of the tank 80. The tanks 80 may be made of a suitable metal, such as a heavy grade of copper. A suitable size of tank 88 in a tank 80 that has internal dimensions of about 1200 mm by 800 mm in plan view is about 100 mm diameter. The tanks 88 are connected to the potable water supply by suitable valves and controls (not shown) and are provided as a backup supply for peak periods. FIG. 8 shows a axial cross-sectioned view of one of these tanks 88. each tank has a diameter 89 ranging from about 50 to 150 mm and a height 91 ranging from about 800 to 1500 mm. An inlet pipe 93 has an exit 95 near the base 97 of the tank through which potable water enters the tank 88. An outlet 99, located in the top 101 of the tank is provided to remove hot water.

Indirect hot water systems have a maximum continuous flow rate, which is the maximum flow of potable water that can be passed through the system without the outlet temperature falling below a set value. This is dependent on a number of factors, including the rate at which heat is transferred from the static water in the tank to the potable water in the heat exchanger, the rate at which heat is transferred from the refrigerant to the static water and the inlet temperature of the potable water.

Once the water flow exceeds the maximum continues flow rate, the temperature will fall below the set value. If the maximum heat flow to the potable water from the static water exceeds the maximum heat flow into the static water from the refrigerant then the static water will slowly cool. If the maximum heat flow to the potable water from the static water is less than the maximum heat flow into the static water from the refrigerant then the static water will remain at its maximum temperature. This is, obviously a mater of choice as to the design of the heat exchangers.

Where the potable water heat exchanger or the potable water inlet temperature limits the maximum water flow, increasing the heat transfer rate of the potable water heat exchanger or the inlet temperature will increase the maximum continuous flow rate possible.

When the demand for hot water is low, the static water is maintained at its maximum temperature and the water in the storage tanks is heated to and maintained at that temperature. In the event that the demand for hot water is such that the water flowing though the heat exchanger cannot be heated to the required temperature, water may be taken from the tanks 88, either totally or together with water flowing through the heat exchanger. Over a short period of “excess” flow, the static water will not cool greatly and so will not cool the water in the storage tanks 88 greatly. Obviously potable water will flow into the storage tanks 88 and after most of the hot water in the tanks has been removed, the water in the tanks 88 will be relatively cold. By monitoring the temperatures of the potable water at the outlets of the heat exchanger and the storage tanks 88, the system can cease taking water from the storage tanks 88 when the temperature of water from the storage tanks 88 is less than the temperature of water from the outlet of the heat exchanger.

Once the demand for hot water falls below the maximum continuous sustainable flow, the temperature of the static water and the water in the storage tanks 88 will return to the set maximum. An alternative arrangement is that the storage tanks 88 are in series with the potable water heat exchanger. Potable water will flow into the heat exchanger and then into the storage tanks 88 before exiting the system. Thus there will always be a quantity of hot water that maybe extracted at that steady state temperature, irrespective of the flow rate.

Referring to FIG. 2 there is shown an indirect hot water heating system 100 according to an embodiment of the invention. The system 100 acts as an add-on to an existing refrigeration system and replaces or supplements the heat dissipation side of the refrigeration system. Accordingly the system 100 receives a refrigerant gas 103 to a high pressure and temperature from a conventional refrigeration system at 104. The hot refrigerant gas flows into an inlet 106 of a heat exchanger 108 of an indirect water heating tank 110. The heat exchanger 108 has a series of windings, not shown, and is similar to the heat exchanger 22 of FIG. 1. The refrigerant typically arrives at the inlet 106 at a temperature of about 21° F. (about 98.9° C.).

The tank 110 contains a body of static fluid 112, usually water, that is heated by the refrigerant as it passes through the heat exchanger 108. The refrigerant leaves the heat exchanger at outlet 114 at about 95° C. The temperature of the static fluid varies depending of potable water flow but is usually held in a range of 65° C. to about 85° C. Potable water passes through a second heat exchanger 116 that is immersed in the static fluid 112. The potable water first passes through the secondary heat exchanger 130 and then enters the inlet 118 of the heat exchanger 116 at a lower temperature than the static fluid, is heated as it passes through the heat exchanger 116 and leaves at a higher temperature at outlet 120 usually in the range of about 65° C. to about 85° C. If one or more storage tanks 88 of FIG. 7 are in series with the heat exchanger 116, the water passes through these tanks 88 before arriving at the outlet 120.

The heat exchanger 116 may be a multi-coil heat exchanger such as shown in FIGS. 1. Preferably the coils of heat exchanger 116 are supported and separated by the spacer clips shown in FIGS. 3 to 6. The coils of the heat exchanger 116 may be copper or a plastics material. Polyethylene and polypropylene are suitable materials, being rated to about 120° C. and 150° C. respectively.

The refrigerant leaves the tank 110 and passes to a secondary heat exchanger 130 for heating of potable water upstream of the inlet 118 of the tank 110. Mains pressure potable water enters at 135 and flows through the core of the heat exchanger. The secondary heat exchanger is a reverse flow jacket type heat exchanger in which a jacket 132 surrounds a section of the potable water supply pipe 134. The flow of potable water in the pipe 134 is opposite to that of the refrigerant in the jacket 132. Whilst the drawing shows a single potable water pipe 134 within the jacket 132, it will be appreciated that more than one pipe or tube for the potable water may pass through the jacket 132. The potable water pipe(s) may be straight pipe(s) or they may be coiled. The inlet temperature of the potable water in Australia usually ranges from about 15° C. in winter to about 20° C. in summer. The refrigerant enters the heat exchanger 130 at inlet 136 at a temperature from about 85° C. to about 95° C. and heats the water. The potable water leaves the heat exchanger 130 at exit 137 a temperature from about 10° C. to 15° C. higher than at entry, i.e. at about 25° C. to 35° C. The refrigerant leaves the heat exchanger at outlet 138 at a temperature from about 50° C. to about 60° C. and returns to the refrigeration system. Typically the refrigerant is then cooled to dissipate excess heat to the environment and is then expanded through an expansion valve before being used to cool air, such as for air conditioning, before returning to the compressor. Since the system 100 is, in effect, an alternate heat dissipation apparatus, it may be used with any heat pump systems that generate a heated fluid at a sufficiently high temperature. It will be appreciated that the source of hot fluid is not limited to a heat pump and other source, such as solar cells or panels may be used.

The use of the two heat exchange subsystems 110 (the indirect water heating tank) and the secondary heat exchanger 130 significantly increase the amount of heat transferred to the potable water.

In prior art systems the refrigerant only transfers heat to the potable water in the tank 110 and so the heat transferred is small, due to the high temperature of the static water. An electric heater, such as item 19 in FIG. 1 is thus needed to provide additional heat when the refrigerant cannot supply sufficient heat to the potable water. In prior art systems the refrigerant typically only transfers about 20% of the total heat load and the electric heater supplies additional heat to the potable water. In this context the heat load is the heat extracted by the compression and expansion cycle plus the heat input (by the compressor) to drive the cycle.

In the present invention additional heat is transferred to the potable water from the secondary heat exchanger 130 so that significantly less electrical energy is required. It is calculated that with suitable heat exchangers, the secondary heat exchanger 130 is capable of transferring more heat to the potable water than the indirect heating tank apparatus. It is estimated that a system according to the invention may transfer 50% or more of the total heat load to the potable water. Because of the increased overall efficiency of the two heat exchanger apparatus, where an electric heater is provided to heat the static water, it is preferable that this is only operational when the refrigeration system is not active. This may be achieved by a simple relay that disconnects the electric heater whenever the compressor(s) of the refrigeration or air conditioning plant are operating.

Referring to FIGS. 9 to 12 there is shown a tank 200 according to embodiments of the invention. The tank has an inner wall 202 and an outer wall 204. The walls have a thickness of about 6 mm and a separation of about 50 mm. The walls are shaped at their upper end 203 to define a sideways extending slot 210 to receive a gasket. FIG. 11 shows a first embodiment in which the inner wall 202 extends vertically and at its upper end has an outwardly extending annular wall portion 206. The outer wall 204 also extends vertically and at its upper end has an inwardly extending annular wall portion 208 preferably the wall portion is integrally formed with the inner wall. The outer wall 204 does not extend upwards as much as the inner wall 202 and so the portion 208 lies beneath the portion 206, defining a horizontal annular slot 210 therebetween. In use, a ring shaped gasket 212 is provided that has a sealing portion 214 and a retaining portion 216. The retaining portion includes a two horizontally extending annular walls 218 and 220 that define slot 222 therebetween. The gasket is a single piece moulding. The lower wall 218 is sized to fit snugly in the slot 210. The slot 222 between walls 218 & 220 is sized to snugly receive the annular wall portion 206. The gasket is sized so that it is in tension and so pulls itself tightly into the annular slot 210. The wall portion 208 is also provided with apertures 224 in which nuts or the like are located for securing the lid to the tank. Preferably the wall is moulded until the nuts in position during manufacture. In use, the lid is tightened against the gasket, deforming the sealing portion 214. When it is necessary to remove the lid for inspection or the like, the fasteners are removed and the lid is raised. Because the gasket is firmly secured to the horizontal tongue and groove arrangement, the gasket is less likely to become detached from the tank.

FIG. 12 shows a second embodiment of the top of the tank that provides similar functionality. The inner wall 250 is formed with two outwardly extending annular wall portions 252 and 254 at its upper end. The two wall portions 252 and 254 are spaced vertically from each other and so define a horizontal slot 256 for receiving the gasket 212 as shown in FIG. 11. Referring to FIGS. 9 and 10, the tank 200 is provided with vertically extending ribs 230 on the inner surface of the inner wall 202. The ribs 230 may be integrally formed with the inner wall 202 or may be separate items secured to the inner wall 202 by glue or fasteners, not shown. In the preferred form the inner wall and ribs are roto-moulded simultaneously to form an integral unit. In the embodiment shown four ribs 230 are provided, each at 900 to each other. The ribs 230 extend from adjacent the base 232 to adjacent the upper end 203.

The base 232 is also provided with four ribs 234 that extend horizontally from adjacent the centre of the base to near the inner wall 202. The central portion 236 is sized to provide clearance for a circle of about 100 mm and aids in water flow around the heat exchangers.

The ribs 234 are also arranged at 90° to each other and in the embodiment shown extend adjacent the vertical ribs 230. This is not essential and the horizontal ribs need not align with the vertical ribs 230.

The ribs 230 and 234 serve to space the heat exchanger coils away from the base and side wall of the tank and so aid movement of the static water past the windings of the two coils. This increases the maximum heat flow possible. The horizontal ribs 234 also reinforce the base of the tank.

The outer wall of the tank also includes an annular groove 238 to aid in grasping the tank when installing or otherwise moving the tank.

FIG. 13 is a schematic diagram showing an indirect hot water heating system according to a further embodiment of the invention. The system is substantially the same as that shown in FIG. 2 and accordingly the same components have the same numbers.

The system of FIG. 13 is designed to improve the efficiency in winter, when the system is not running at full capacity and incorporates a climate control unit 302, located downstream of the secondary heat exchanger 130 and before the compressor unit 300.

The climate control unit 302 maintains a substantially constant pressure at the outlet 138 of the secondary heat exchanger 130. this in turn maintains a substantially constant condensing temperature, summer or winter. This is achieved by having an automatically adjusting valve that varies the restriction to flow in a valve of the climate control unit 302. In winter, the compressor unit 300 does not run at full capacity and only a heat flow is required to provide cooling. Accordingly, only a small heat flow is available to heat the water. Without the climate control valve, the temperature of the refrigerant exiting the compressor unit 300 is substantially less in winter than in summer. Accordingly, there is only a small temperature difference between the refrigerant and the water in the secondary heat exchanger 130. Thus the temperature increase of the water as it passes through the secondary heat exchanger 130 is not great. This is particularly the case where there is not a great flow of potable water through the secondary heat exchanger. In this circumstance the potable water will reach the maximum possible for the temperature of the incoming refrigerant. By increasing the temperature of the incoming refrigerant, the temperature of the potable water may be increased, so increasing the amount of heat transferred to the water.

The climate control unit 302 limits the amount of refrigerant flowing, so in winter, rather than having a large flow of refrigerant exiting the compressor 300 at a low temperature, the climate control valve causes the system to have a small flow of refrigerant exiting the compressor 300 at a high temperature.

Thus, in both summer and winter, refrigerant leaves the compressor 300 and travels along supply pipe 104 at about 110° C. to about 116° C. The refrigerant passes through the primary heat exchanger 108 and heats the potable water to about 65° C. to about 75° C. The refrigerant leaves the primary heat exchanger at about 95° C. and is still mainly vapour. The refrigerant enters the secondary heat exchanger 130 and is cooled by the potable water to be substantially liquid. The potable water leaves the secondary heat exchanger 130 at about 50° C. to 55° C., although this depends on the flow of potable water. Because the refrigerant enters the secondary heat exchanger 130 at about 95° C. summer or winter, there is the potential to heat the potable water to a higher temperature than otherwise. When there Is no flow of potable water, the water in the secondary heat exchanger will be heated to near the temperature of the refrigerant, so the hotter the refrigerant, the hotter the potable water.

FIG. 14 shows a means to boost the heat recovery capacity beyond what is presently possible with conventional indirect heated hot water systems but without using a secondary heat exchanger. This configuration is of more benefit to small installations where the capital cost of the secondary heat exchanger unit of FIGS. 2 and 13 cannot be justified.

Similar components are used in FIGS. 14 and 2 and accordingly the same components have the same identifying numbers.

The system of FIG. 14 has a single water tank 110 in which heat is transferred from refrigerant flowing through a first heat exchanger 108 to the static water 112 in the tank and hence to a potable water supply via second heat exchanger 116. Refrigerant leaves the first heat exchanger at 114 and passes directly to the climate control unit 302 via line 136. The climate control unit 302 controls pressure and hence refrigerant fluid flow as described with reference to FIG. 13. The refrigerant then flows to the condenser 303 before returning to the heat pump (not shown). The effect of the climate control unit 302 is to reduce refrigerant flow in so the refrigerant leaves the primary heat exchanger at approximately 90° C., with the body of static water 112 heated to about 85° C. and the potable water heated to about 80° C. at the outlet 120.

FIG. 15 shows a heat exchange tank 310, such as that shown in FIG. 1 having an electrically operated water level sensor 312. The heat exchange tank 310 has a body of static water 313, which acts as a heat transfer medium between the two heat exchange coils, not shown for clarity. As the water is heated, over time there will be some loss to the environment and the water level needs to be checked periodically. External sight tubes may be used but these are liable to damage due to accident or vandalism. The water level may be checked by removing the cover of the tank, but this is time consuming and again vandals bay remove the lid. The tank 310 of FIG. 14 incorporates a tube 316 located internally in the tank. The tube is mounted on the inner surface 317 of the tank. The tube 316 is open at both ends to the water within the tank and so the level of the water surface within the tube corresponds to that in the tank. A float 314 is located within the tube 316 that incorporates a magnetic component 318. A magnetically operated reed switch 320 is mounted adjacent the tube 316 and positioned at a height corresponding to a minimum acceptable water level. In normal use the water level is above the reed switch and the float is remote from the reed switch. The reed switch is normally (open or closed?). When the water level drops, the float falls to be adjacent the reed switch and changes the state of the switch to be (closed or open?). An electric circuit connected to the reed switch detects the change of state and may activate an audible or visual alarm. It will be appreciated that if the water level continues to fall the float will eventually become out of range of the reed switch and the reed switch will change state. To ensure that the alarm only turns off when the water level rises, the system may be provided with a second read switch 322 located above the first read switch. The alarm system may be configured to only turn the alarm signal off when the second reed switch 322 turns off after the first reed switch, i.e. corresponding to a rising water level.

It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive. 

1. A heat exchange system that transfers heat between a first fluid and a second fluid, the system including: a first heat exchanger including: a first inlet for the first fluid; a second inlet for the second fluid; a first outlet for the first fluid; a second outlet for the second fluid; and a constant pressure valve located downstream of the first heat exchanger and having a fifth inlet in fluid communication with the first outlet of the first heat exchanger that maintains a substantially constant pressure at the fifth inlet.
 2. The system of claim 1 including a second heat exchanger having: a third inlet for the first fluid; a fourth inlet for the second fluid; a third outlet for the first fluid; a fourth outlet for the second fluid, wherein, the third inlet is located downstream of the first outlet and the second inlet is located downstream of the fourth outlet.
 3. The system of claim 2 wherein the constant pressure valve is located downstream of the third outlet.
 4. The system of claim 1 wherein the first heat exchanger is an indirect heat exchanger utilising a third fluid to transfer heat from the first fluid to the second fluid.
 5. The system of claim 1 wherein the first heat exchanger includes: a tank; a third fluid within the tank; at least one first pipe extending between the first inlet and first outlet and extending at least partially within the third fluid, and at least one second pipe extending between the second inlet and second outlet and extending at least partially within the third fluid.
 6. The system of claim 5 including at least one hollow reservoir within the first tank, said at least one reservoir being at least partially immersed in the third fluid, the interior of at least one reservoir being in fluid communication with at least one of the second inlet and the second outlet.
 7. A heat exchange system that transfers heat between a first fluid and a second fluid, the system including: a first heat exchanger including: a tank; a third fluid within the tank; a first inlet for the first fluid; a second inlet for the second fluid; a first outlet for the first fluid; a second outlet for the second fluid; at least one first pipe extending between the first inlet and first outlet and extending at least partially within the third fluid, and at least one second pipe extending between the second inlet and second outlet and extending at least partially within the third fluid; and a second heat exchanger including: a third inlet for the first fluid; a fourth inlet for the second fluid; a third outlet for the first fluid; a fourth outlet for the second fluid, wherein, the third inlet receives first fluid from the first outlet and the second inlet receives second fluid from the fourth outlet.
 8. The system of claim 7 including a constant pressure valve located downstream of the first heat exchanger and having a fifth inlet in fluid communication with the first outlet of the first heat exchanger for maintaining a substantially constant pressure at the fifth inlet.
 9. The system of claim 8 wherein the constant pressure valve is located downstream of the third outlet.
 10. The system of claim 7 further including at least one hollow reservoir within the first tank, said at least one reservoir being at least partially immersed in the third fluid, the interior of at least one reservoir being in fluid communication with at least one of the second inlet and the second outlet.
 11. A heat exchange system that transfers heat between a first fluid and a second fluid, the system including: a first heat exchanger including: a first inlet for the first fluid; a second inlet for the second fluid; a first outlet for the first fluid; a second outlet for the second fluid; and a second heat exchanger including: a third inlet for the first fluid; a fourth inlet for the second fluid; a third outlet for the first fluid; a fourth outlet for the second fluid, the third inlet receiving first fluid from the first outlet and the second inlet receiving second fluid from the fourth outlet, and a constant pressure valve located downstream of the first heat exchanger and having a fifth inlet in fluid communication with the first outlet of the first heat exchanger for maintaining a substantially constant pressure at the fifth inlet.
 12. The system of claim 11 wherein the constant pressure valve is located downstream of the third outlet.
 13. The system of claim 11 wherein the first heat exchanger is an indirect heat exchanger utilising a third fluid to transfer heat from the first fluid to the second fluid.
 14. The system of claim 11 wherein the first heat exchanger includes: a tank; a third fluid within the tank; at least one first pipe extending between the first inlet and first outlet and extending at least partially within the third fluid, and at least one second pipe extending between the second inlet and second outlet and extending at least partially within the third fluid.
 15. The system of claim 14 further including at least one hollow reservoir within the first tank, said at least one reservoir being at least partially immersed in the third fluid, the interior of at least one reservoir being in fluid communication with at least one of the second inlet and the second outlet. 